2. Description of the Related Art
Photolithographic projection objectives have been in use for several decades for producing semiconductor components and other finely structured structural elements. They serve the purpose of projecting patterns of photomasks or reticles, which are also denoted below as masks or reticles, onto an object coated with a photosensitive layer with very high resolution on a reducing scale.
Three developments running in parallel chiefly contribute to the production of ever finer structures of the order of magnitude of 100 nm or below. Firstly, an attempt is being made to increase the image-side numerical aperture (NA) of the projection objectives beyond the currently customary values into the region of NA=0.8 or above. Secondly, ever shorter wavelengths of ultraviolet light are being used, preferably wavelengths of less than 260 nm, for example 248 nm, 193 nm, 157 nm or below. Finally, still other measures are being used to increase resolution, for example phase-shifting masks and/or oblique illumination.
In addition, there are already approaches to improving the achievable resolution by introducing an immersion medium of high refractive index into the space between the last optical element of the projection objective and the substrate. This technique is denoted here as immersion lithography. The projection objectives suitable for this purpose are denoted as immersion objectives or immersion systems. Introducing the immersion medium yields an effective wavelength of λeff=λ0/n, λ0 being the vacuum operating wavelength and n the refractive index of the immersion medium. This yields a resolution of R=k1 (λeff/NA0) and a depth of focus (DOF) of DOF=±k2 (λeff/NA02), NA0= sin Θ0 being the “dry” numerical aperture, and Θ0 being half the aperture angle of the objective. The empirical constants k1 and k2 depend on the process.
The theoretical advantages of immersion lithography reside in the reduction of the effective operating wavelength and the resolution improved thereby. This can be achieved in conjunction with an unchanged vacuum wavelength, and so established techniques for producing light, for selecting optical materials, for coating technology etc. can be adopted largely without change for the appropriate wavelength. However, measures are required for providing projection objectives with very high numerical apertures in the region of NA=1 or above. Furthermore, suitable immersion media must be available.
Ultra-pure water with n1≈1.43 emerges as a suitable immersion medium for 193 nm.
The article entitled “Immersion Lithography at 157 nm” by M. Switkes and M. Rothschild, J. Vac. Sci. Technol. Vol.19 (6), November/December 2001, pages 1 ff. presents immersion fluids based on perfluoropolyethers (PFPE) which are sufficiently transparent for a working wavelength of 157 nm and are compatible with some photoresist materials currently being used in microlithography. One tested immersion fluid has a refractive index of n1=1.37 at 157 nm. The publication also describes a lens-free optical system, operating with calcium fluoride elements and silicon mirrors, for immersion interference lithography, which is intended to permit the projection of 60 nm structures and below in conjunction with a numerical aperture of NA=0.86. The optical system may not be suitable for use in the series production of semiconductors or the like.
Patent Specifications U.S. Pat. No. 4,480,910 and U.S. Pat. No. 5,610,683 (corresponding to EP 0 605 103) describe projection exposure machines, provided for immersion lithography, having devices for introducing immersion fluid between the projection objective and the substrate. No design is specified for the optical projection system.
Some projection objectives suitable for immersion lithography have recently become known. Purely refractive projection objectives known from the international patent applications WO 03/077036 and WO 03/077037 A1 (corresponding to US 2003/30174408) of the applicant are designed as so-called single-waist systems or two-belly systems with an object-side belly, an image-side belly and a waist situated there between, that is to say a constriction of the beam bundle diameter. Image-side numeral apertures of up to NA=1.1 have been achieved in this case.
Attempts to achieve yet higher apertures are rendered difficult because the maximum lens diameters increase dramatically as apertures become larger, and this complicates the fabrication of the projection objectives, making it more expensive. In addition, the chromatic aberrations and here, in particular, the lateral chromatic aberrations assume disturbing values. The lateral chromatic aberrations (CHV) are also denoted as chromatic magnification aberrations, and have the effect that partial images are imaged with a different size for different wavelengths. The consequence of this is that the lateral chromatic aberration does not occur on the optical axis, but is noticed ever more strongly toward the edge of the image field (field dependence).
Chromatic aberrations are usually reduced by using at least two optical materials of different dispersion inside a projection objective. However, in the wavelength region of the deep ultraviolet (DUV) at operating wavelengths of less than 200 nm only a few transparent optical materials with sufficiently low absorption are available. For applications at 193 nm, use is made chiefly of synthetic silica glass (fused silica) (SiO2) as principal material and, as second type of material, fluoride crystal materials such as calcium fluoride (CaF2) or barium fluoride (BaF2). As a rule, at 157 nm calcium fluoride is used as principal material and barium fluoride as second material. However, said fluoride crystal materials are available only to a limited extent, expensive and difficult to work. Consequently, it is desired to have optical designs that manage with only one type of material, in particular with synthetic silica glass. In any case, the chromatic aberrations must be minimized such that contrast losses caused by the chromatic aberrations remain tolerable when use is made of radiation sources that are suitably narrowband. Particularly important here is the correction of the lateral chromatic aberration, since the latter produces a contrast loss that is a function of the field.